1. Field of the Invention
This invention pertains to detonation initiation in a combustible material by imploding shocks generated by impinging jets in a chamber defined as combustor filled with the combustible material.
2. Description of Related Art
Detonation is a very efficient combustion process that couples chemical energy release to shock waves, generating extremely high pressures. Therefore, propulsion devices based on detonation can operate at higher pressure levels, hence, greater propulsion efficiency than conventional propulsion engines based on the constant-pressure combustion process such as flame or deflagration. Among the detonation-based propulsion devices, the pulse detonation engine looks particularly promising. Pulse detonation engine is a propulsion device using the high pressure generated by repetitive detonation waves in a combustible material. For most pulse detonation engines, the operating frequency is 50 Hz to 1000 Hz, corresponding to operating cycle time of 0.02 to 0.001 seconds. Detonation initiation in pulse detonation engines is one of the most challenging problems in the development of pulse detonation engines.
Traditional methods of detonation initiation, such as direct initiation or deflagration to detonation transition, are impractical for practical pulse detonation engine applications. In the direct initiation process, a significant amount of energy is applied to the combustible material by energy-depositing devices, such as high-power spark plugs or lasers, to directly initiate detonation. However, the amount of energy required for direct initiation of the conventional combustible material used in pulse detonation engines is impractically large . In the deflagration to detonation transition process, a small amount of energy is used to ignite a flame or deflagration in the combustible material which later transitions into a detonation as it propagates through the combustible material. The main difficulty with using deflagration to detonation transition for pulse detonation engine applications is that the transition distance is too long for a practical pulse detonation engine system.
There have been persistent efforts to overcome the initiation difficulty by either lowering the initiation energy requirement in the direct initiation process or reducing the transition distance in the deflagration to detonation transition process. Internal blockages or obstacles, such as spirals, have been introduced into the pulse detonation engine tube to shorten the deflagration to detonation transition distance with limited success. However, the blockage parts in the pulse detonation engine tube negatively impact the pulse detonation engine performance and significantly complicate the engine configuration. Another approach is to use chemical additives, such as oxygen or very energetic hydrocarbons, to reduce the initiation energy requirement to a level that can be provided by practical energy-depositing devices, such as spark plugs or lasers. However, carrying additional fuel additives is undesirable for aviation applications.
U.S. Pat. No. 5,473,885 to Hunter et al is entitled “Pulse Detonation Engine” describes a pulse detonation engine which has a detonation chamber with a sidewall and two fuel ports located in the sidewall. In this design, an oxygen-fuel mixture is introduced through the forward port and detonated, creating a detonation wave propagating into an air-fuel mixture introduced through the rearward port. This patent primarily focuses on the pulse detonation cycle and detonation tube and related valve structures.
U.S. Pat. No. 5,800,153 to DeRoche entitled “Repetitive Detonation Generator” describes an apparatus and method for generating detonation waves. In the patented apparatus, the detonation is generated by electric spark plugs in a tube. However, besides showing some spark plugs in the system schematics, the patent neither provides any specifics on the spark plug ignition system in particular nor makes any claim in methods or devices for detonation initiation in general.
U.S. Pat. No. 5,937,635 to Winfree et al entitled “Pulse Detonation Igniter for Pulse Detonation Chambers” describes a pulse detonation engine with a pulse ignition system and a plurality of detonation chambers. The main feature of this design is the use of the igniter for multiple detonation tubes or chambers. The ignition system comprises several small tubes and detonation waves are initiated in oxygen-enriched mixture in those tubes by electric spark plugs or lasers. The major disadvantages of this design include system complication and the high power requirement by electric spark plugs or laser energy depositor; additional system complications for handling the added oxygen, which is especially disadvantageous to aviation engines; and difficulties during detonation transition from a small initiation tube, where the detonation is generated by spark plug or a laser, to a detonation tube of a larger size. During the transition, the detonation may fail.
Reference paper AIAA 02-3627 entitled “Initiation Systems for Pulse Detonation Engines,” by Jackson and Shepherd describes a initiation method in which multiple small detonations arm combined to a focusing region to generate a detonation covering the entire pulse detonation engine tube. However, in this approach, the small detonations are still needed to be initiated by spark plugs and a complex tubing system is required to synchronize arrival times of the small detonations at the focusing region.